Dynamic down-regulation of surface expression of activated α2 integrins in platelet precursors in response to collagen. (A) Expression of surface α2 in GFP⁺ CD41⁺ cells harvested from mice reconstituted with hematopoietic stem cells lacking both α2 and GPVI exposed to empty vector (vector) or vector expressing wild-type α2 integrins (α2WT), α2E318A integrins, and α2GFFKRΔ integrins are shown. The gray shadowed area represents α2 levels in GFP⁻ nontransfected cells. Top panels show expression immediately after harvest from the bone marrow, middle panels show expression after 72 hours of ex vivo culture in the absence of collagen, and bottom panels show expression after 72 hours of ex vivo culture on collagen-coated plates. (B) Quantitation of α2 surface expression in the GFP⁺ CD41⁺ cells described in panel A. N = 5; mean and SD are shown.